Flat panel X-Ray detectors are in wide use in medicine. Most of these flat panel X-Ray detectors are based on a single light detector chip coupled with a scintillator. However, detectors of such a design are typically expensive. The single detector chip may be replaced by a plurality of less expensive optical sensors (e.g. CCD or CMOS) and lenses, which are arranged into a flat multi-camera array. X-Ray detectors including the multi-camera array may be less expensive in comparison with the single chip detectors since simpler sensors and lenses may be used. In multi-camera X-Ray detectors, each optical sensor acquires optical light irradiation from a segment of the scene as radiated from the scintillator. A complete image may be composed by stitching the plurality of partial images acquired by the plurality of single sensors.
The output image quality may be measured and assessed visually by visibility of the seam between the stitched partial images. Unfortunately, two neighbor images typically have intensity discrepancies in overlapping regions caused by differences in sensors and lenses. One of the sources of image distortion is geometric distortions caused by the lenses. Unfortunately, each lens causes different geometric distortions.
According to prior art, geometric distortions may be corrected by placing a set of markers, such as a mechanical Cartesian grid, on top of the detector (the top side being the side facing the imaged body and the side at which the images are captured), during a calibration phase. The mechanical Cartesian grid may include, for example, metal sheet with substantially identical holes drilled at known and typically evenly spaced locations. X-Ray radiation is applied and an image of the grid is created on the scintillator. Each sensor acquires its portion of the scintillator image, with certain geometric distortion. The locations of the holes on the grid are known. The locations of the same holes on the distorted partial images are found using digital image processing techniques. A transformation for correcting geometric distortion is calculated based on mapping between the location of the holes on the grid and the location of the same holes on the distorted images.
Correction of geometric distortion using the method described hereinabove requires that the grid be placed on top of the scintillator directly, without any significant space between them. However, real life detectors are typically enclosed in a casing. Thus, placing the grid directly over the scintillator requires opening of the casing of the detector for calibration. However, opening of the casing of the X-Ray detector is undesirable since it can, for example, lead to contamination of the system.
On the other hand, if the grid is placed on the casing of the detector, at a distance from the scintillator, the X-Ray source geometry may cause parallax. As a result, the projection of the grid holes on the scintillator will have parallax distortion. Hence, an image of the grid will suffer from parallax distortion on top of the geometric distortion. A geometric distortion correction function derived from such an image may be inaccurate. Using such a correction function may lead to undesired distortion in the final X-ray image. The parallax distortion is independent and uncorrelated with the geometric distortion.
The magnitude of parallax distortion largely depends on a ratio between the distance from the grid to the scintillator and the distance from the scintillator to the X-Ray source. As this ratio increases (for example, as the distance from the scintillator to the X-Ray source increases), the magnitude of parallax distortion increases. Errors in estimation of the geometric distortion correction function can be minimized by placing the grid very close to the scintillator or by placing the X-Ray source at a large distance from the detector. In real life settings, however, both actions frequently cause problems.
It would, therefore, be desirable to calculate a geometric distortion correction function that is minimally influenced or even not influenced by the parallax distortion, without opening the cover of the detector.